docs: add literate prose to naming-drift aliases and HITL gateway changes

Adds prose sections before every code block to satisfy the
prose-before-code discipline. Each backward-compatibility alias
(process-signal, perceive-gate, reason-gate, act-gate, inject-stimulus)
now has its own subsection explaining why it exists and what new code
should use instead.

Also:
- Fixes double #+end_src in core-loop-perceive.org
- Renames inject-stimulus → stimulus-inject in heartbeat-start and
  client-handle-connection (both already had aliases)
- Adds HITL interception prose to gateway-manager.org telegram/signal
  sections
- Splits Pre-Reason Handler Registry into two code blocks (defvar + defun)
  for one-per-block compliance

org/gateway-manager.org

@@ -33,6 +33,11 @@ Registration of available gateway implementations: each platform registers its p
 #+end_src
 
 ** Telegram Implementation
+
+When a Telegram message arrives, the gateway first checks whether it is a
+HITL approval/denial command via ~hitl-handle-message~. If consumed,
+the message never enters the cognitive pipeline. Otherwise, it is injected
+as a normal ~:user-input~ event via ~stimulus-inject~.
 ;; REPL-VERIFIED: 2026-05-03T13:00:00
 #+begin_src lisp
 (defun telegram-get-token ()
@@ -94,6 +99,10 @@ Registration of available gateway implementations: each platform registers its p
 #+end_src
 
 ** Signal Implementation
+
+Signal messages follow the same pattern as Telegram: ~hitl-handle-message~
+is called first, and only non-HITL messages are injected into the pipeline.
+
 ;; REPL-VERIFIED: 2026-05-03T13:00:00
 #+begin_src lisp
 (defun signal-get-account ()